Tooth mineral is composed predominantly of calcium hydroxyapatite, Ca10(PO4)6(OH)2, which may be partially substituted with anions such as carbonate or fluoride, and cations such as Zn and Mg. Tooth mineral may also contain non-apatitic mineral phases such as octacalcium phosphate and calcium carbonate.
Tooth loss may occur as a result of dental caries, which is a multifactorial disease where bacterial acids such as lactic acid produce sub-surface demineralisation that does not fully remineralise, resulting in progressive tissue loss and eventually cavity formation. The presence of a plaque biofilm is a prerequisite, and acidogenic bacteria such as S. mutans may become pathogenic when levels of easily fermentable carbohydrate, such as sucrose, are elevated for extended periods of time.
Even in the absence of disease, loss of dental hard tissues can occur as a result of acid erosion and/or physical wear; these processes are believed to act synergistically. Exposure of the dental hard tissues to acid causes demineralisation, resulting in surface softening and a decrease in mineral density. Under normal physiological conditions, demineralised tissues self-repair through the remineralising effects of saliva. Saliva is supersaturated with respect to calcium and phosphate, and in healthy individuals saliva secretion serves to wash out the acid challenge, and raises the pH so as to alter the equilibrium in favour of mineral deposition.
Dental erosion is a surface phenomenon that involves demineralisation, and ultimately complete dissolution of the tooth surface by acids that are not of bacterial origin. Most commonly the acid will be of dietary origin, such as citric acid from fruit or carbonated drinks, phosphoric acid from cola drinks and acetic acid such as from vinaigrette. Dental erosion may also be caused by repeated contact with hydrochloric acid (HCl) produced by the stomach, which may enter the oral cavity through an involuntary response such as gastroesophageal reflux, or through an induced response as may be encountered in sufferers of bulimia.
Tooth wear is caused by attrition and/or abrasion. Attrition occurs when tooth surfaces rub against each other, a form of two-body wear. An often dramatic example is that observed in subjects with bruxism, a grinding habit where the applied forces are high, and is characterised by accelerated wear, particularly on the occlusal surfaces. Abrasion typically occurs as a result of three-body wear and the most common example is that associated with brushing with a toothpaste. In the case of fully mineralised enamel, levels of wear caused by commercially available toothpastes are minimal and of little or no clinical consequence. However, if enamel has been demineralised and softened by exposure to an erosive challenge, the enamel becomes more susceptible to tooth wear. Dentine is much softer than enamel and consequently is more susceptible to wear. Subjects with exposed dentine should avoid the use of highly abrasive toothpastes, such as those based on alumina. Again, softening of dentine by an erosive challenge will increase susceptibility of the tissue to wear.
Dentine is a vital tissue that in vivo is normally covered by enamel or cementum depending on the location i.e. crown versus root respectively. Dentine has a much higher organic content than enamel and its structure is characterised by the presence of fluid-filled tubules that run from the surface of the dentine-enamel or dentine-cementum junction to the odontoblast/pulp interface. It is widely accepted that the origins of dentine hypersensitivity relate to changes in fluid flow in exposed tubules, the hydrodynamic theory, that result in stimulation of mechanoreceptors thought to be located close to the odontoblast/pulp interface. Not all exposed dentine is sensitive since it is generally covered with a smear layer, an occlusive mixture comprised predominantly of mineral and proteins derived from dentine itself, but also contains organic components from saliva. Over time, the lumen of the tubule may become progressively occluded with mineralised tissue. The formation of reparative dentine in response to trauma or chemical irritation of the pulp is also well documented. Nonetheless, an erosive challenge can remove the smear layer and tubule “plugs” causing outward dentinal fluid flow, making the dentine much more susceptible to external stimuli such as hot, cold and pressure. As previously indicated, an erosive challenge can also make the dentine surface much more susceptible to wear. In addition, dentine hypersensitivity worsens as the diameter of the exposed tubules increases, and since the tubule diameter increases as one proceeds in the direction of the odontoblast/pulp interface, progressive dentine wear can result in an increase in hypersensitivity, especially in cases where dentine wear is rapid.
Loss of the protective enamel layer through erosion and/or acid-mediated wear will expose the underlying dentine, and are therefore primary aetiological factors in the development of dentine hypersensitivity.
It has been claimed that an increased intake of dietary acids, and a move away from formalised meal times, has been accompanied by a rise in the incidence of dental erosion and tooth wear. In view of this, oral care compositions which help prevent dental erosion and tooth wear would be advantageous.
Oral care compositions generally contain a source of fluoride ions for promoting remineralisation of teeth and for increasing the acid resistance of dental hard tissues. To be effective the fluoride ions must be available for uptake into the dental hard tissues being treated. However some components of dentifrices such as silica abrasives can interact with and thereby decrease the amount of freely available fluoride ions.
One solution to this problem is suggested in U.S. Pat. No. 4,340,583 (Huber) which describes a toothpaste composition comprising a source of fluoride ions formulated with a specially treated silica abrasive that has good fluoride ion compatibility. These compositions have a pH in the range of 4 to 8, preferably 6.5 to 7.5 and may advantageously contain a soluble phosphate pellicle film penetration agent, a preferred example being an orthophosphate salt. Example 11 describes a low abrasive level toothpaste containing, as surfactant, a sodium alkyl sulphate.
Another solution is suggested in WO 01/66074 (Colgate) which recommends that the fluoride source should be stored at alkaline pH (to prevent interaction with a silica abrasive which may occur at acidic pH) but delivered at an acidic pH so as to enhance the uptake of the fluoride ions into the tooth enamel. This is achieved by the use of a dual component dentifrice, one phase being alkaline and containing the fluoride ions, the other phase being acidic and containing phosphate ions, which on mixing prior to use, provides an acidic phosphate fluoride composition (pH 4 to 6).
WO 04/012693 (Colgate) describes a dental composition having heightened desensitisation as well as heightened tooth fluoridation and remineralisation which is apparently achieved by combining a fluoride ion with a potassium salt, the composition having a pH in the range 7.5 to 9 and being buffered with an alkali metal phosphate salt.